1. Technical Field
The present invention relates generally to power conversion and in particular to uninterruptible power supplies (UPSs). Still more particularly, the present invention relates to a secondary power supply with a battery charging circuit for providing an uninterruptible power system and a method of operation thereof.
2. Description of the Related Art
Uninterruptible power supplies are required for many applications where the loss of a primary power source has serious implications. For example, where computer systems are utilized to transfer large amounts of data over a network, it is essential that a backup, or secondary, power supply is available in the event that the primary power source is lost to prevent the loss of valuable information.
Early uninterruptible power supplies (UPSs) were placed between a standard AC utility outlet and an AC input socket of an electrically powered device. These power supplies typically included an AC to DC power converter for converting a received AC power into DC power to charge a battery and a DC to AC converter for reconverting the DC power back into AC power for supply to the AC input socket of the electrical device. This resulted in considerable duplication of circuitry as an AC to DC power converter was also typically included in an integral power supply of the electrical device.
Current conventional UPS systems typically employ a rectifier to convert an AC input power to DC power that, in turn, is subsequently inputted to a power factor correction stage, such as a DC/DC boost converter, that raises the DC power voltage level prior to providing the raised DC voltage to a plurality of DC/DC converters that may include a switching regulator and a bias regulator. The output voltages generated by the bias regulator are generally utilized by control circuitry within the UPS and the outputs of the switching regulator is typically employed to provide power to any load device that is connected to the UPS. The secondary or backup power source is typically a battery or a battery bank, depending on the application and/or load power requirements. The backup battery is conventionally placed at the input power stage of the DC/DC converters and is electrically coupled to the DC/DC converters with a switching device to provide power in the event of a loss of primary power. The switching device is controlled by a controller that monitors the AC input power. The switching device along with its required control circuitry are part of the cost considerations in the design of the secondary power supplies in UPS systems.
Furthermore, unlike the batteries employed in telecommunication applications, in server and workstation applications, the secondary power source, i.e, batteries, are typically lead acid batteries that need to be charged at a certain controlled rate to prevent heating which reduces the operational life of the batteries. Furthermore, the float voltage of the batteries also needs to be well regulated to ensure that the batteries are properly charged. In conventional UPS systems, the battery is typically charged using a flyback type of rectifier followed by an inverter stage that is used to provide 50/60 Hz power to the power system. This charging scheme is expensive to implement and not cost-effective for low cost non-mission critical applications.
Accordingly, what is needed in the art is an improved uninterruptible power supply. In particular, what is needed in the art is a more cost-effective design implementation for secondary power supplies employed in UPS systems.